Institutional Review Board (IRB) approval of this research could not be verified during an investigation by the Landesärztekammer Rheinland-Pfalz (the State Medical Association of Rheinland-Pfalz in Germany). As a result, this article has been retracted, in accordance with the Journal's regulations, at the request of the Directors and Board of the British Journal of Anaesthesia. Lack of IRB approval means that the research was unethical, and that IRB approval for the research was misrepresented in the published article. It does not mean that the research results per se are fraudulent. For further information please see Retraction.